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3 Answers
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There is a case when you can use the future perfect to describe an estimation (more than a probability):
"Había muchísima comida. Habré comido unos diez bocadillos" (="There was a lot of food. I must have eaten about ten sandwiches").
The difference is, when using those tenses by referring to the past:

- Future perfect can be used for estimating something that, albeit known that happened for sure as it is a lived experience, cannot be assessed accurately (like in above example).

- Conditional perfect can be used a possibility of what could have happened if some conditions have had met. (e.g. "Habría comido diez bocadillos si me hubiesen dejado" -> "I would have eaten ten sandwiches if they had allowed me")

So, answering to your second question, use the future perfect when estimating something, and the conditional perfect when assessing a possibility. The difference is subtle, but as a rule of thumb think in the first case when the fact you are talking about has indeed happened, and in the second case when the fact you are taking about could have happened, or there is more incertainty in what you are saying.

So present perfect is completed at some point prior to the present: he comido, he hablado.

Pluperfect is completed at some point prior to the past: Ya había comido cuando llegó. Ya había pensado antes de hablar. Hence it's called the past of the past.

Likewise, future perfect is completed at some point prior to the future time frame. You might well call it the past of the future. habré viajado a diez países antes de jubilar (future time frame: upon retirement, and prior to that, but posterior to now, ten countries will be visited).

Conditional is, technically, the future of the past as evidenced by indirect speech: Me dice que irá a Méjico because Me dijo que iría a Méjico when put in past tense. In Spanish, the conditional tense is even called the pospretérito.

So conditional perfect is, if you will, the past of the future of the past. It happens before the future that is established within a past-tense timeframe. (mind blown yet?)